The present invention relates to secondary loop refrigeration, and in particular, to a method and apparatus using as a secondary loop refrigerant, tetrafluoroethane also commonly known as R-134a.
The cooling system for commercial and retail establishments generally comprise a remotely located primary unit that is individually connected to the various cooling loads or zones therein, such as air conditioning, low temperature freezer units, and mid-temperature refrigeration units. Such arrangements in a typical supermarket refrigeration system oftentimes require hundreds or thousands of pounds of refrigerant charge in addition to thousands of feet of refrigerant lines. Additionally, plural primary units may be employed, however, each conditioned area nonetheless requires individual connection.
The problems associated with the above approaches have been further complicated by changes in the permissibility and availability of direct expansion refrigerants commonly used for such systems. Certain chlorofluorocarbons and perfluoroalkanes are being phased out because of their environmental impact. To the extent obtainable, the cost of such refrigerants are increasing markedly making the cost of the installed system considerably more expensive. Certain non-chlorinated low temperature and medium temperature refrigerants have been developed as alternatives, however, they tend to be even more costly. Other high temperature direct expansion refrigerants, such as R-134a, are more moderate in cost, but are not effective in direct expansion cooling systems below air conditioning temperatures. At present, accordingly, R-134a finds application predominantly as a direct expansion refrigerant for motor vehicles air conditioning systems.
The foregoing problems have prompted refrigeration equipment manufacturers to propose the use of secondary liquid cooling. Therein, a primary condensing unit is closely coupled to a direct expansion heat exchanger. The refrigerant for the primary system may be selected based on performance, and because of the shorter supply lines the cost thereof is reduced. The direct expansion heat exchanger is coupled to a secondary system using a liquid secondary refrigerant. The secondary refrigerant is pumped through individual secondary lines to the liquid chilling coils in various temperature control zones, such a refrigerated displays, walk-in coolers and the like.
One such system is disclosed in U.S. Pat. No. 5,713,211 to Sherwood. Therein, a liquid secondary refrigerant is directed in a secondary cooling loop from a primary-secondary heat exchanger to a series of display cases and pumped back to the heat exchanger. Only a single zone, of the many zones typically found in commercial applications, is covered in the secondary loop. The secondary loop is not operative to provide coil defrosting.
Another approach is disclosed in U.S. Pat. No. 5,524,442 to Bergman et.al. wherein a secondary refrigeration loop employs an open loop air stream that directly impinges a product to be cooled. The secondary loop return air system is directed to a secondary heat exchanger interfaced with a primary refrigeration loop.
A plurality of secondary refrigeration loops using a single refrigerant are disclosed in U.S. Pat. Nos. 5,318,845 to Dorini et. al. and 5,138,845 to Mannion et. al. Therein, the return lines of the primary refrigeration are fed in parallel as the inlet lines to the secondary cooling loads and the secondary return lines are connected with the primary inlet lines. Control systems are provided with each cooling load to control temperature and flow rates. While providing some localization of lines, a single refrigerant charge for the cooling demands of the generally similar temperature demands of the various units of the system.
A further approach is disclosed in U.S. Pat. No. 5,042,262 to Gyger et. al. wherein second closed loop system is operative to transfer heat from a single heat sink using carbon dioxide as a secondary refrigerant.
It is apparent from the above that such secondary loop designs have not focused on the major problems associated with plural refrigerant systems, i.e. consolidation of the high cost/high performance primary refrigerant loop and a secondary loop capable of handling plural cooling zones of the type found in supermarkets, cold storage facilities, hospitals, industrial plants, hotels, shopping centers, and like locations requiring cooling, refrigeration and heating. By focusing on parallel exchanges, high fluid volume cost, high equipment costs, and power consumption for fluid transfer remain a problem.
The present invention addresses and overcomes the aforementioned problems and limitations by providing a secondary refrigeration system incorporating a continuous series of progressively increasing temperature zones in a single secondary cooling loop. Therein, R-134a. as a secondary fluid is interfaced with the primary system and has the fluid feed line connected in parallel to a plurality of cooling loads having the highest cooling demands, such as freezer units. The return lines of the first loads are combined and fed to a second zone of cooling loads having the next highest cooling demand, such as refrigerated displays. Thereafter the second zone return lines are fed back to the heat exchanger or to subsequent zones in a similar manner, such as air conditioning equipment.
Such design eliminates the need for individual piping for each zone thereby reducing refrigerant, equipment, power consumption and piping costs. Moreover, the heat exchanger may be bypassed for defrosting the coils in the zones wherein the temperature rise from the line loading will warm the coils sufficiently for defrosting, while upon completion of defrosting, the system may be quickly returned to operative status. Furthermore, the aforementioned design permits the use of low cost non-chlorinated fluids operative in the liquid phase providing the requisite viscosity, specific heat, thermal conductivity, and environmental acceptability while providing efficient heat transfer within temperatures ranging from xe2x88x9240xc2x0 F. to +80xc2x0 F.
Accordingly, it is an object of the present invention to provide a secondary cooling system having reduced material, equipment and operating costs in conditioning a plurality of cooling zones.
A further object of the invention is to provide a plurality of increasing temperature zones that are serially connected in a secondary cooling loop.
Another object of the invention is to provide secondary cooling loop system using environmentally acceptable high performance refrigerants in a liquid phase with chilling coils in a series connection of increasing temperature zones.
Yet another object of the invention is to provide a liquid secondary refrigeration loop connecting a plurality of cooling zones wherein the loop may be quickly and conveniently disabled allowing the latent heat from the units to raise the temperature of the fluid sufficiently for defrosting purposes.